Energy Efficient Windows: 5 ways windows are rated

Energy Efficient Windows

“Energy efficiency” – it sounds so environmentally friendly and fiscally responsible, but what exactly does it mean? Is it just a fancy buzzword, or does it have a quantifiable impact on the quality and performance of our windows?

If energy efficiency is quantifiable, what makes a window energy efficient? Who does the quantifying, and how do they communicate that to the public?

What are the benefits of purchasing an energy-efficient window vs. a non-energy efficient window?

These are some of the questions that come up while shopping for new windows. Let’s break them down and provide the answers you need to make the best decision for you and your home.

What is energy efficiency?

Quite simply, energy efficiency describes a product’s ability to provide the same service using less energy than a similar, non-efficient product.

What does this mean to you? Energy-efficient windows are better at maintaining the temperature inside your home regardless of whether you are in the middle of an arctic freeze or a record heatwave. This helps you to avoid higher utility bills since you won’t need to run your heater or air conditioner as often as if you had a non-efficient window.

Cold vs heat - Energy efficient Window

Who issues energy efficiency ratings?

Two programs/agencies in the United States test and rate windows on their energy performance levels – Energy Star and the National Fenestration Rating Council (NFRC).

A third group – the Efficient Windows Collaborative (EWC) – is an excellent third-party resource for educational materials and minimum NFRC rating recommendations based on what area of the country you live in.

Energy Star

Most of us have heard of Energy Star, even if we are not sure what exactly an Energy Star rating means. The Energy Star program is a joint venture between the Environmental Protection Agency (EPA) and the United States Department of Energy.

Appliances, insulation, windows, lighting, electronics, and heating/cooling products are all tested, and their efficiencies compared to a baseline value. Products that meet a certain threshold are given the Energy Star sticker of approval.

National Fenestration Rating Council (NFRC)

The NFRC is an independent nonprofit organization created by industry leaders. Its purpose is to provide consumers with an objective analysis of the energy performance of a particular window, door, or skylight.

How is energy efficiency quantified?

Both the Energy Star program and the NFRC quantify the energy efficiency performance of products and communicate that data to the rest of us in a standardized way that we can understand.

How do they do that? Let’s find out.

Energy Star keeps its rating pretty simplified. A product either earns the little blue star sticker of approval, or it does not. For a product to be Energy Star approved, it must meet standards for U-factor and solar heat gain coefficient. Each Energy Star sticker also indicates which of the 4 U.S. climate zones the product is rated for.

The NFRC bases its rating on the two factors considered by Energy Star as well as an additional three metrics. This means that a window that is approved by the NFRC will also meet Energy Star standards, but not all Energy Star windows will also be NFRC agreed.

The five factors that the NFRC tests and rates are:


  • Rates the insulation value of a window
  • A lower U-Factor number means that the window is a better insulator.
  • Range: 0.20 – 1.20
  • For northern states like Illinois, the EWC recommends windows that are rated 0.35 or lower.

Solar Heat Gain Coefficient (SHGC)

  • Rates the amount of solar radiation and heat that passes through the window
  • A lower SHGC number means that less radiation and heat are entering your home.
  • Range: 0 – 1
  • For northern states, the recommendation is the highest number possible combined with a low U-Factor if you don’t need to cool your house much in the summer. If you usually run your air conditioner frequently during hot months, choose an SHGC below 0.55.

Air Leakage (AL)

  • Rates the amount of air that passes through the window frame once it is installed. This metric is measured in cubic feet per square foot of window area.
  • A lower number means that the frame is more airtight.
  • Industry-standard: 0.3

Visible Transmittance (VT)

  • Rates the amount of light that the window lets in
  • A lower number means less light is coming in.
  • Range: 0 – 1
  • Reduce glare in a media room by choosing a window with lower transmittance. Craving natural light in your living room or breakfast nook? Opt for panes with a higher visible transmittance rating.

Condensation Resistance

  • Rates the amount of moisture that builds upon the surface of a window or causes glazing between the panes of a dual or triple-pane window
  • A lower number means that the window allows more condensation to build up.
  • Range: 1 – 100
  • Condensation resistance is an optional rating category, and some manufacturers may not include this information.

How are energy efficiency ratings communicated?

When windows meet or exceed the rating standards, manufacturers label them with a certification sticker.

Look for the blue Energy Star sticker and the white NFRC label on the window pane.

What to look for on the window panes

The NFRC label is useful for comparing one window to another because it gives the numerical values the window received in each of the rating categories.

Understanding the NFRC Label

Image source: NFRC

How do the prices of energy-efficient windows compare?

In general, energy-efficient windows tend to cost a bit more initially than less energy-efficient windows.

The trade-off is that over the lifetime of the window, energy-efficient windows will help decrease the number of your energy bills. In contrast, non-efficient windows will lead to higher heating and cooling costs.

Long-term savings from choosing energy-efficient windows

According to Energy Star, replacing old doors, windows, and skylights with energy-efficient options can save you up to 15% annually on your energy bill.

What are the take-aways?

  • Energy-efficient windows help to thermoregulate your home, thereby decreasing your heating and cooling costs.
  • Windows are tested and rated on their energy efficiency by the Energy Star program and the National Fenestration Rating Council (NFRC).
  • Efficient Windows Collaborative helps educate consumers and explain which windows are best for their needs.
  • Energy efficiency is evaluated based on five factors:
    • U-Factor
    • Solar Heat Gain Coefficient
    • Air Leakage
    • Visible Transmittance
    • Condensation Resistance
  • Windows that meet energy efficiency standards are identifiable by their blue Energy Star sticker or white National Fenestration Rating Council label.
  • Energy-efficient windows cost more initially; however, you can expect cost savings in the form of up to 15% lower energy bills over the lifetime of the windows.

Ready to take the next steps towards energy efficiency? Contact us to set up an in-home evaluation.

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